1. Technical Field
This invention relates to the art of designing and fabricating piston assemblies, and more particularly to piston designs that achieve substantially zero clearance with the surrounding cylinder wall within which it operates.
2. Discussion of the Prior Art
This invention addresses problems characteristic of current commercial internal combustion engine piston-cylinder assemblies: excessive crevice volume, premature ring fatigue failure, and excessive blow-by of fluids or induced oil combustion.
Crevice volume is the upper space between the piston and cylinder wall, including the ring groove spaces up to generally the point of sealing of the bottom compression ring; it increases with clearance between the piston crown and bore wall, and increases with groove size. A large crevice volume allows for the presence of unburned fuel to remain in the combustion chamber and thereby increase emissions. This is compounded at cold start when greater fuel is injected into the combustion chamber to initiate and sustain combustion; the resulting unburned fuel will not be readily converted by the exhaust catalyst due to the cold start conditions of the catalyst. It is well to keep in mind that the design of the piston relative to the cylinder bore wall is conventionally set for the smallest clearance at the maximum speed/load condition; therefore thermal expansion of the piston material relative to the bore wall material (i.e. aluminum pistons to a cast iron bore wall) will cause the crevice volume to increase the cold start condition.
It would be ideal to have a piston that reciprocates within a cylinder bore wall with no clearance between the piston crown and the bore wall and with little or no friction under all operating conditions. However, to attain durability of the interfacing materials, they have been restricted to those that may not produce the lowest friction, such as iron or steel coated with nickel or chromium for the piston rings, iron or aluminum for the bore wall which sometimes is coated with wear resistant coatings, and iron or aluminum for the piston skirt which sometimes is coated with wear resistant coatings. All of these coatings must be stable at extreme temperature cycling such as between -20.degree. to 400.degree. F. Accordingly, lower friction materials stable at lower temperatures may not be considered suitable to reduce crevice volume.
Premature fatigue failure of piston rings can be caused by high gas pressure bottoming out the compression rings in their grooves, while the piston slaps against the bore wall, thus jarring and stressing the frozen rings counter to their tension while they are dragged against a nonconforming cylinder wall. Since reciprocating forces change magnitude and direction every 180.degree. (and a major change at firing pressure every 720.degree.), such stressing constitutes impact loading of the rings. Impact loading leads to groove wear, ring instability (commonly referred to as flutter), and eventually ring failure by fatigue. Such rings can also get extremely hot accentuated by high friction of the rings within their grooves, allowing microwelding to take place in some instances. It would be desirable if a better thermal path was available, other than through the rings, for heat to be extracted from the piston and conveyed to the cylinder bore wall for ease of removal by the cooling jacket.
Blow-by or migration of combustion gases or fluid oil past the piston rings is a continuous problem for piston assembly design. Fluids can migrate from the combustion chamber past the back side, front side or through the split ends (commonly referred to as "end gap") of the piston rings; The ring dynamics described above, combined with these leakage paths, is usually accompanied by poor oil film scraping allowing oil to migrate upward into the combustion chamber resulting in contamination by deposits on the combustion chamber walls. Blow-by of combustion gases to the crank case, reduces engine compression and robs the engine of its designed power. More often than not, such leakage, either upwardly or downwardly past the piston rings, is augmented by high friction of the piston rings within their grooves. The piston crown wall interfacing with the cylinder bore wall has not been used to restrict blow-by heretofore.